Guillermo Bustos-Pérez \(^{1,2,3}\), Javier Baena \(^{1}\), Manuel Vaquero \(^{2,3}\)
\(^1\) Universidad Autónoma de Madrid. Departamento de Prehistoria y Arqueología, Campus de Cantoblanco, 28049 Madrid, Spain
\(^2\) Institut Català de Paleoecologia Humana i Evolució Social (IPHES), Zona Educacional 4, Campus Sescelades URV (Edifici W3), 43007 Tarragona, Spain
\(^3\) Universitat Rovira i Virgili, Departament d’Història i Història de l’Art, Avinguda de Catalunya 35, 43002 Tarragona, Spain
Abstract
Lithic artefacts are usually associated with the different knapping methods used in their production. Flakes exhibit metric and technological features representative of the flaking method used to detach them. However, lithic production is a dynamic process in which discrete methods can be blurred, and in which features can vary throughout the process. An intermediate knapping method between the discoid and Levallois is commonly referred to under an umbrella of terms (the present research uses the term hierarchical discoid), and is associated with a broad geographical and chronological distribution throughout the Early and Middle Palaeolithic. This intermediate knapping strategy exhibits features of both the discoid and Levallois knapping methods, raising the question of the extent to which flakes from the three knapping methods can be differentiated and, when one is mistaken for another, the direction of confusion. An experimental assemblage of flakes detached by means of the three methods was used along with an attribute analysis and machine learning models in an effort to identify the knapping methods employed. In general, our results were able to very effectively differentiate between the three knapping methods when a support vector machine with polynomial kernel was used. Our results also underscored the singularity of flakes detached by means of Levallois reduction sequences, which yielded outstanding identification values, and were rarely erroneously attributed to either of the other two knapping methods studied. Mistaking the products of the discoid and hierarchical discoid methods was the most common direction of confusion, although a good identification value was achieved for discoid flakes and an acceptable value for hierarchical discoid flakes. This shows the potential applicability of machine learning models in combination with attribute analysis for the identification of these knapping methods among flakes.
Keywords: lithic technology; experimental archaeology; Levallois; discoid; Middle Palaeolithic; machine learning
Extended abstract
La producción de lascas se asocia a diferentes métodos de talla. Las lascas resultantes presentan características métricas y atributos que son representativos del método de talla del que se han producido. Sin embargo, la talla lítica es un proceso dinámico en el que los métodos de talla definidos pueden verse entremezclados debido a adaptaciones a las características volumétricas y de calidad de la materia prima, diferentes fases a lo largo del proceso de reducción, aspectos cronoculturales, etc. Esto da lugar a que las características de los productos de talla varíen a lo largo del proceso de reducción. Bajo diferentes términos es común encontrar alusiones a un método de talla intermedio entre el discoide y el Levallois, presentando una amplia distribución geográfica y cronológica a lo largo del Paleolítico Medio y el Paleolítico Medio inicial. La concepción de este método de talla, referido en el presente documento como Discoide Jerárquico, posee características intermedias entre el Levallois (jerarquización de superficies no intercambiables o un plano de talla paralelo a la intersección de ambas superficies) y el discoide (ausencia de preparación de talones, planos de talla secantes en la fase inicial de talla), surgiendo la duda de hasta qué se pueden diferenciar los productos de lascado de los tres métodos y sobre la direccionalidad de las confusiones.
El presente trabajo emplea un conjunto experimental de lascas procedentes de los tres métodos de talla (77 del método de talla discoide, 73 del Levallois y 72 del Discoide Jerárquico). Sobre este conjunto experimental de lascas se realiza un análisis métrico y de atributos, y sobre los datos procedentes de este análisis se entrenan diez algoritmos de aprendizaje automático con el objetivo de determinar hasta qué punto es posible diferenciar el método de talla. Para evaluar los algoritmos de aprendizaje automático se tiene en cuenta la precisión general de los modelos, pero también los efectos del uso de umbrales de probabilidad en la identificación de los métodos de talla. El uso de umbrales de probabilidad permite optimizar el ratio de positivos verdaderos y positivos falsos para cada umbral de decisión y de ahí extraer el “área bajo la curva” (AUC en inglés) como valor de avaluación de un modelo.
De los diez algoritmos de aprendizaje automático, una máquina de vector soporte con kernel polinomial presenta los mejores resultados en la identificación de los tres métodos de talla, proporcionando unos resultados excelentes a la hora de diferenciar entre los tres métodos a nivel general (0.667 precisión, 0.824 AUC). Considerando individualmente cada método de talla, los resultados subrayan el carácter singular de las lascas procedentes de secuencias de reducción Levallois ya que obtienen una identificación excepcionalmente buena (AUC de 0.91), siendo su procedencia raramente atribuida a cualquiera de los otros dos métodos. La confusión entre productos procedentes de secuencias de talla discoide y el Discoide Jerárquico es más común, aunque se alcanza una identificación excelente en el caso de los productos procedentes de reducciones discoides (AUC de 0.82) y una identificación aceptable en el caso los productos procedentes del Discoide Jerárquico (AUC de 0.73).
Estos resultados muestran el potencial de combinar modelos de aprendizaje automático con análisis de atributos sobre lascas para la identificación de métodos de talla. Su uso puede servir de gran ayuda en la identificación de métodos de talla en lascas. Sin embargo, su uso requiere de una evaluación previa de los conjuntos líticos para determinar posibles métodos de talla existentes, uso diferencial de las materias primas, y evaluación de las cadenas operativas presentes.
Palabras clave: tecnología lítica; arqueología experimental; Levallois; Discoid; Paleolítico Medio; Aprendizaje Automático
The Middle Palaeolithic in western Europe is characterised by the increase in and diversification of prepared core knapping methods, resulting in flake-dominated assemblages (Kuhn 2013). These flake-dominated assemblages are the result of a wide number of production methods including Levallois (Boëda 1994; Boëda 1995b; Boëda et al. 1990), discoid (Boëda 1993; Boëda 1995a), the système par surface de débitage alterné or SSDA (Forestier 1993; Ohel et al. 1979), Quina (Bourguignon 1996; Bourguignon 1997), different laminar production systems (Boëda 1990; Révillon & Tuffreau 1994), and the Kombewa (Newcomer & Hivernel-Guerre 1974; Tixier & Turq 1999) among several others. This abundance of different production methods results in a highly diversified material culture in which flakes exhibit great morphological variability. Flakes often retain morphologies and attributes characteristic of the knapping method used to detach them, facilitating the identification of those methods. However, flakes also often present overlapping attributes and morphologies as a result of the high internal variability of the methods used and the fact that flakes with similar functional properties can be produced via different methods Kuhn (2013). Due to their extensive geographical and chronological distribution, the Levallois and discoid constitute important sources of cultural variability in the Middle Palaeolithic of western Europe.
Boëda (1994; 1995b) establishes six characteristics defining the Levallois knapping strategy from a technological point of view:
Depending on the organization of the debitage surface Levallois cores are usually classified into preferential method (were a single predetermined Levallois flake is obtained from the debitage surface) or recurrent methods (were several predetermined flakes are produced from the debitage surface) with removals being either unidirectional, bidirectional or centripetal (Boëda 1995b; Boëda et al. 1990; Delagnes 1995; Delagnes & Meignen 2006).
Because of its early recognition in the XIX century (Boucher de Perthes 1857), its association with cognitive abilities of planning and predetermination (Boëda 1994; Pelegrin 2009), and its use for the definition of cultural facies (Bordes 1961a; Bordes 1961b) and lithic technocomplexes (Delagnes et al. 2007; Faivre et al. 2017), the Levallois flaking technology is considered a trademark of the Middle Paleolithic. Emergence of the Levallois method is observed from MIS12 to MIS9, with several sites presenting elements characteristic of Levallois production(Carmignani et al. 2017; Hérisson et al. 2016; Moncel et al. 2020; Soriano & Villa 2017; White & Ashton 2003). However, Levallois is clearly generalized and identified from MIS8 onwards, covering a wide geographical distribution throughout Western Europe (Delagnes et al. 2007; Delagnes & Meignen 2006; Faivre et al. 2017; Geneste 1990). The long geographical and temporary span of Levallois adds additional layers of variability which can result from raw material constraints, synchronic variability as a result of different site functionality, chronological trends in development of methods or shifts in the technological organization of groups. Attention is also called on the explicit recognition of Levallois cores after MIS 8, while a multitude of terms is employed to define previous hierarchical knapping strategies and its possible coexistence with Acheulean technocomplexes (Moncel et al. 2020; Santonja et al. 2016; Hérisson et al. 2016; Rosenberg-Yefet et al. 2022; White & Ashton 2003; Scott & Ashton 2011).
Boëda (1993; 1994; 1995a), also establishes six technological criteria defining the Discoid method:
Technological analysis of Middle Paleolithic assemblages has gradually led to identify a variability of modalities within the discoidal core knapping (Bourguignon & Turq 2003; Locht 2003; Terradas 2003; Locht 2003). This has resulted in sensu stricto and a sensu lato conceptualizations of the Discoid knapping system (Faivre et al. 2017; Mourre 2003; Thiébaut 2013). The sensu stricto highly corresponds to Boëda’s (1993) above mentioned definition, where core edge flakes and pseudo-Levallois points are the most common products. The sensu lato Discoid encompasses a larger range of products (although centripetal flakes are more common) as a result of higher variability in the organization of percussion and exploitation surfaces (Terradas 2003).
One of the variants from the Discoid sensu lato conceptualization resembles Levallois knapping strategy (Figure 1). Some common characteristics outlined for this method are:
1) The core volume is conceived as two hierarchical asymmetric surfaces: the percussion surface and the exploitation surface (this is a common feature with Levallois).
2) Preparation of the percussion surface is absent or it is partial, without encompassing the complete periphery of the core. This can be a result of raw material characteristics presenting an adequate morphology or because it is achieved with minimal preparation.
3) Despite the hierarchical nature of both surfaces flakes detached from the debitage surface present a secant relationship towards the plane of intersection. Soriano and Villa (2017) call attention that Levallois products usually present an external platform angle (EPA) between 80º and 85º, while products from non-Levallois hierarchical methods present an EPA relationship between 70 and 85º. However, this relationship can change along the core’s reduction with final flakes being sub-parallel to the plane of fracture (Slimak 1998).
4) Products from Hierarchical Discoid are usually symmetrical towards the knapping direction, are thin, and the ventral and dorsal surfaces present a subparallel relation. Again, these are common traits with Levallois products.
Schematic representation of the knapping methods, surfaces and platform preparation
Strategies from several sites can be considered to fit the above mentioned variation of discoidal knapping method and its resemblance to the Levallois method has been previously noted for several Middle and Early Middle Paleolithic assemblages (Casanova i Martí et al. 2009; 2014; Jaubert 1993; Peresani 1998; Slimak 1998; 2003; Soriano & Villa 2017). However, it is important to consider that given the wide geographical and chronological span (Figure 2), different terms are employed. For Middle Paleolithic sites, the identity of this method usually focuses on the shared features with Levallois and Discoid and thus, its intermediate nature.
Jaubert (1993) at Mauran notes the hierarchical nature of the production system and its resemblance to exhausted recurrent centripetal Levallois cores. However, he points out the secant planes of detachment not so parallel as Levallois as a differentiation. Slimak (1998; 2003) at Champ Grand also notes the similarities of residual cores with recurrent centripetal Levallois debitage. Casanova i Martí et al. (2009; 2014) ) notes for Estret de Tragó the presence of products and knapping methods which share features of Levallois and Discoid, and proposes to include Hierarchical Discoid and Levallois recurrent centripetal strategies into the Hierarchical bifacial centripetal class. Peresani (1998) for Fumane cave notes the presence of debitage products with features (reduced thickness, debitage angle, and centripetal organization of scars also subparallel to the ventral surface) which would correspond to parallel planes debitage. Baena et al., (2005) indicate the presence of hierarchical Discoid along the sequence of Esquilleu cave as secondary and primary knapping method.